metals & alloys Flashcards
what are metals and alloys used for in dentistry?
RPD framework crowns denture base orthodontic appliance restorations
what is the main drawback of metals?
poor aesthetics
what is the advantage of metals?
superior mechanical properties
- strength
- rigidity
- hardness
- elastic limit
- some ductility
what is a metal?
aggregate of atoms in crystalline structure
what is an alloy?
combination of metal atoms in crystalline structure
what is ductility?
amount material will deform without fracture - subject to tensile stress
what property is demonstrated if compressive stress applied?
malleability
what does the crystalline structure of a metal depend on?
history- method of production
shaping- crucial for dental applications eg. cold working
which factors affect mechanical properties?
crystalline structure
grain size
grain imperfections
what are the 3 lattice arrangements?
cubic, face-centred cubic, body centred cubic
how are grains formed?
molten state above melting point cools changes state plateau- liquid and solid solid cools
describe crystal growth
first atoms act as nuclei of crystallisation
crystals grow to form dendrites
crystals (grains) grow until they impinge on other crystals
region where grains make contact called grain boundary
what is it called if crystal growth is of equal dimension in each direction?
equi-fixed grains
what is radical grain structure?
molten metal cooled quickly in cylindrical mould
what is fibrous grain structure?
wire pulled through die (narrow circular aperture)
-cold worked metal/alloy
what is the result of fast cooling (quenching)?
more nuclei
small fine grains
what is the result of slow cooling on crystal growth?
few nuclei
large coarse grains
what are nucleating agents?
impurities or additives act as foci for crystal growth
what is a grain boundary?
change in direction of the crystal planes
why are small grains advantageous?
high elastic limit
increased FS and UTS, hardness
what is the disadvantage of small grains?
decreased ductility
what are the factors needed to quench?
small bulk
heat metal/alloy just above melting point before cooling
store in mould/container that readily conducts heat
how do defects affect crystal/grain?
weaken grain so overall structure
grain not perfect
bond less effective than perfect ones
if force applied, defect more likely to be fractured
what is the result if force applied to defect?
defect moves along lattice plane
- crystal shape changes
- characteristics of metal change
- until reaches grain boundary so grain has all atoms inter-connected with same strong bond
what effect does impeding movement of dislocations have on metals/alloys
increases: -elastic limit -fracture stress/UTS -hardness decreases: -ductility -impact resistance
what factors impede dislocation movement
grain boundaries
alloys-dif atom sizes
cold working
-dislocations stopped at grain boundaries
what is cold work?
change shape by bending rolling etc.
done at low temp (below recrystallisation temp)
causes slip so dislocations collect at grain boundaries
so stronger, harder material
what does cold work increase?
elastic limit
fracture stress/UTS
harness
residual stress
what does cold work decrease?
ductility
impact strength
corrosion resistance
what is the impact of residual stress?
causes instability in lattice
results in distortion over time
undesirable
what is residual stress relieved by?
annealing
what is annealing?
heating m/a so that greater thermal vibrations allow migration of atoms
re-arrangement allows any instability/distortion w/i lattice to be eliminated
when is recrystallisation used?
when cold working hasn’t had desired effect?
what is recrystallisation?
m/a heated causing: -smaller, equiaxed grains -lower EL, UTS, hardness -increased ductility re-set allows further cold work can repeat until correct shape obtained
what determines recrystallisation temp?
depends on amount of cold work
greater cold work-> lower temp needed
what effect does excessive temp rise cause on grain growth?
large grains replace smaller course grains yielding poorer mechanical properties
